USGS IN YOUR STATE

SITE INSPECTIONS AND INVESTIGATIONS

1. General. Site investigations consisted of detailed
visual inspections and photographs of the dam site, soil sampling,
excavations of portions of the remaining embankment, plane table surveys of
the site, and investigation of the debris downstream of the dam by
the Board members and data gathering teams composed of personnel
from all the Federal agencies involved. Discussion will be divided
between investigations at the site and inspections of debris
downstream of the dam.

2. Observations at the Dam Site.

In order to facilitate site inspections, a plane table survey
was conducted
and a base line was
established. The location of the base line is shown on the
plan view (Figure 1). An assumed datum has been used which is
540.46 feet above National
Geodetic Vertical Datum (mean sea level). Investigations
consisted of detailed visual inspections, measurements, soil
sampling, and physical property testing of
various materials found at the site.

The dam site is located on Toccoa Creek, approximately 2000
feet upstream of Toccoa Falls, on the northwest side of the
City of Toccoa, Georgia. The dam crest width varies but the
average width is approximately 20 feet. Side slopes vary but
near the failure area appear to be about 1V on 1H. Maximum
height is approximately 38 feet. The total length of the dam
was approximately 400 feet, although only a section about 200
feet long appears to be higher than 20 feet. This is the
section in which the failure occurred (Photos 5 and 6). The embankment is a homogeneous
earth section in that apparently no selectivity was made of
material coming from the borrow pits. The
predominant embankment material is residual soil consisting
mainly of silts of varying plasticity and
some silty sands and clays.
The embankment materials are distinguishable from the
foundation materials by their reddish and tan colors. There is
no evidence of an internal drain or foundation cutoff.
Portions of the embankment were constructed across a layer of
alluvial material (silt). This layer is clearly visible in the photograph (Photo 9). The
remaining embankment slopes are heavily vegetated and trees up
to one foot in diameter are common (Photo 10). Near the ends
of the dam the vegetation makes it difficult to distinguish
between the fill section and
natural ground.

There exists an uncontrolled earth spillway (main earth
spillway) excavated through the hillside beyond the leftabutment (Photo 7).
The
spillway is about 60 feet wide at the entrance but quickly
narrows to a base width of approximately 11 feet for the major
portion of its length. It is about 380 feet long and has side
slopes of about 1V on 1H.
The invert appears to vary from
about 10 feet below the crest, at the upstream side, to about
20 feet at the discharge area. The spillway area is heavily
grown up, predominantly by small trees and brush along the
spillway sides. One person interviewed stated that this
spillway was constructed in the 1960's. However, aerial photos
taken January, 1955, clearly show the spillway. In addition to
this spillway, there exists a low spot in the reservoir rim, on
the right side, in the
vicinity of the "Fort Toccoa" sign that has
effectively served as a secondary spillway (Photo 11).
There is evidence that water has gone over the rim at this
location several times in the recent past including the night of
the failure. The crest of the secondary spillway is seven feet
below the top of the dam.

Debris marks on the upstream face of the dam indicate that
there was five feet of freeboard below the earth
road at the dam crest prior to failure. The water depth in the
main earth spillway was about four feet. There is a report
that the fireman who inspected the dam site two to four hours
before it failed found no water in the secondary spillway.
Another
unconfirmed report has the lake level falling.

3. Data Revealed by the Failure.

The left side of the
breach exhibited tension cracks
extending about 30 feet back from the failure scarp. The cracks were
perpendicular to the dam axis and decreased in width from the
edge of the breach. The face of the breach had a slope of
about 1V on 1H (Photo 8). The central portion of the crest of the remaining dam was free
of vegetation except for grass. The slopes and outer edges
were covered with trees and brush. The brush on the upstream
edge of the crest was free of debris. The grass on the crest,
extending from the breach back to the spillway, was not knocked
down or bent from the vertical. Also noted on the crest was an
old campfire site and numerous newspapers, all undisturbed (Photo 5). On the downstream slope of the left
embankment, the location
of a rotted out tree stump was observed. This had left a
vertical hole in the embankment. Also on the downstream side
of the left breach, water had washed away the leaves up to
about 21 feet above the existing channel. The after-failure
survey indicates the high-water marks. The
left bank water mark is about 14 feet higher than the
high-water mark on the right
side of the channel. These marks are indicated on the site topographic plan (Figure 3, 156k). Some old rock and rubble were
noted on the downstream toe of
the left breach. This material looks as if it was placed,
rather than being a natural deposit. Also noted were several
logs or timbers, likely the remnants of old cribbing.

Spillway inspections indicate the main earth spillway
usually carried approximately one foot of water. The fact that
water continually flowed over the main earth spillway is
supported by staining on the rock at the spillway outlet and
statements of historical witnesses. Topsoil and leaves were
scoured off of the spillway sides up to about four feet above
the bottom of the spillway; however, three to four feet of
spillway freeboard
apparently remained during the failure.

The breach exposed the dam's foundation under both sides of
the breach (Photo 5). On the extreme right side, the flood had
cut to rock. The erosion of the foundation revealed about four
feet of alluvialsilt beneath the embankment and
overlying the rock (Photo 9). The exposed face (scarp) is practically vertical
on the right side (Photo 10). From the left edge of the stream
to the toe of the left scarp, the exposed silt layer slopes
upward. There appears to be two distinct and different layers
of silt beneath the fill. The bottom layer, immediately over
the rock, is about two feet thick near the embankment
centerline and is a tan plastic silt with some sand. (The tan
color may be due to some oxidation). Overlying this layer is
about two to three feet (at centerline) of grey fat micaceous silt. At the
material interfaces, there are remnants of old root systems.
Along the root line, seepage was noted after
failure. The impression is that the lower layer was natural
material and the grey layer the result of later deposition such
as would occur in a reservoir. At least in some locations, the
embankment was built across both these layers. The layers are
less pronounced near the downstream toe of the embankment.

The right side of the breach was considerably steeper than
the left. The scarp was practically vertical for a
considerable distance and the overall slope is estimated at 1V
on 0.5H. As previously observed in the left scarp, a
considerable root system is exposed and extended down into the
embankment (Photo 10). These roots varied in size up to three
inches in diameter and some extended down into the remains of
the rock crib dam. No
tension cracks were observed on the right embankment crest.
The vertical distance from the exposed rock in the streambed to
the top of the dam on the right side of the failure is
approximately 42 feet. This includes about four feet of silt
and 38 feet of fill material. On the downstream exposure of
the right breach, there is a flat area similar to a berm. (This is not evident on
the left side). This berm appears to be located about half way
up the embankment from the toe. An area of rock and rubble is
exposed beneath the downstream toe of the right scarp face
about 73 feet downstream of base line station 2+60 (Photo 10). The rock has about an 18-inch maximum size. This rubble
is similar to the material observed under the left toe except
that there is a much larger quantity of material. The area is
approximately 12 feet wide at the base and six feet high. The
material extends to natural rock. On the upstream side of the
material a feature appearing to be a grout filled sandbag was
observed. In the vicinity of the rock and rubble, about 78
feet downstream of base line station 2+75, a log about 12 feet
long and 18 inches in diameter was found (Photo 10). This log
was secured to the underlying rock by a steel pin one-inch in
diameter. This log is believed to be a remnant of the old
cribbing. Also noted in this area was another steel pin which
was sheared off at the top of rock level and appeared to have
been abandoned for some time. Other holes, apparently
drilled, were observed in the foundation rock.

Immediately above and upstream of the rubble area a 30-inch
diameter spiral riveted steel pipe was found with its open
flange face roughly corresponding to the face of the failure
surface (Photo 9). This pipe is about 60 feet downstream of
base line station 2+60. The invert of the pipe roughly
corresponded to the top of the underlying silt layer. The pipe
runs sub-parallel to
the dam axis and daylights in the downstream slope where it
appears to have been cut off and abandoned. Metalurgical
analyses date the pipe as being installed about 1900. The
pipe is partially collapsed inside. Also noted was a vertical
pipe about four inches in diameter penetrating the 30-inch pipe
from the top and protruding about a foot into the larger pipe.
The small pipe was plugged at the top. The flanged end of the
30-inch pipe shows no evidence that the pipe was sheared.
Rather, it appears the pipe was unbolted at some time in the
past and left open. The upper half of the pipe is surrounded
by a piece of concrete about a foot or so from the end. This
concrete may have been part of a headwall, seepage collar, or other
old structure. At the flanged end of the pipe, wood framing
and cribbing is exposed. This material was left in place when
the earth fill was
constructed.

Approximately 25 feet upstream from base line station 2+20,
remnants of what appears to be a concrete sill were found on and in the
face of the embankment.
The top of the sill was about 30 feet below the embankment
crest. On the top of the sill there were remnants of concrete
masonry walls. This sill would have been the base of the
welded steel penstock
inlet for the pipe to the powerhouse. Excavations into the
area of the scarp at the
location of the masonry sill, revealed a 30-inch welded steel
pipe which had a wall thickness of 3/32-inch. The pipe had a
square flange of 1/4-inch thick steel plate. The pipe, later
exposed by excavation, was uncoated. About half the flange was
torn away from the end of the pipe. The pipe was partially
collapsed but inspections indicate that this collapse was not
recent. The alignment of the pipe is such that it appears to
lead down the mountain to the old Toccoa Falls Power Plant. No
evidence of piping or
erosion was noted around the
pipe.

It is important to note that all remaining slopes of the
dam were heavily vegetated. Trees in excess of one-foot in
diameter are common. The breach surface indicates a
significant
root system down into the embankment.

Observations on the right
downstream slope of the dam revealed the 30-inch spiral riveted
steel pipe exposed on the downstream toe about 85 feet from its
exposure in the breach. The pipe appears to have been cut off
and abandoned. The pipe is about 80 percent plugged with soil,
but leaves and topsoil were washed away at the mouth of the
pipe. It appears that some water went through the pipe for a
short period of time during the failure once the end of the
pipe was exposed on the
scarp.

Below the dam there exists remnants of two lines of pipe
leading to the Toccoa Falls Power Plant. The older line is
spiral riveted steel pipe such as exposed in the right
embankment toe at the rubble area. The newer line is at a
higher elevation and is welded steel pipe. This newer pipe
aligns with the pipe excavated near the concrete sill upstream
of the base line. The new
line also exhibits an air vent tower
and gate valve as shown in Figures 2 and 3.

The area below the dam was inspected in order to observe
debris. It was believed that the debris observations might give
some clue as to the dam history and construction. The following
observations were made:

Pieces of metal were found. These included what
appeared to be a track, gate wheel, trash grate, metal
channel, steel plate, and various pieces of pipe of varying
diameters, some being bituminous coated.

Large logs and timbers such as had been observed to be
bolted to the rock at the site were found. These were about
12-inches to 18-inches in diameter and contained old
spikes.

Several pieces of concrete masonry were found. In
general, these pieces ranged from approximately three to
four feet in each direction. They appear to be remnants of
an old concrete block wall, perhaps a portion of the outlet
structure. One piece was curved as if it had fitted around
a pipe. The diameter of the curve was about 30 inches.

A steel plate structure was observed which contained
remnants of a sliding gate over an outlet pipe. The pipe
diameter was about 30 inches. The plate was three sided
with dimensions of about three feet by four feet. It
appeared to have been some type of liner for a masonry
structure due to imprints on the exterior and masonry in
the area.

4. Sampling and Testing

In an effort to obtain information relative to the history
and condition of the dam prior to failure, samples were
obtained of embankment
and foundation soils and certain pieces
of pipe found in the area.

Soil samples were obtained from the site for purposes of
testing the materials in order to have an indication of the
properties of the dam. Samples of various materials from a
number of locations were obtained for classification tests.
Two bulk or chunk type undisturbed samples were taken from the
two predominant foundation materials. The upper grey silt
layer sample was taken from about 38 feet below the crest and
about eight feet downstream of base line station 2+26. A
sample of the underlying residual foundation soil was obtained
from near base line station 2+72, about 40 feet below the
embankment crest. Also, undisturbed Shelby tube type samples
were obtained from a core boring located at base line station
2+00 on the
crest of the right breach.

Testing of soil samples was conducted at the Corps of
Engineers South Atlantic Division Laboratory, Marietta,
Georgia. Testing was conducted in accordance with Corps of
Engineers Manual EM 1110-2-1906, Laboratory Soils
Testing. Classification and moisture content tests were
performed on all materials. Shear tests were performed
on the undisturbed samples. Field density and
standard compaction
tests were performed on certain of the materials from the
Shelby tube embankment samples. Pin hole dispersion
tests
were performed on the grey foundation silt and embankment
materials to determine if the material was highly erodable or
susceptible to piping.
Unconfined and rapid direct shear tests were performed to
compare peak and ultimate strengths, as well as
strain at peak strength and at failure.

Tests indicate the embankment was predominantly composed of
fine grained soils which classify as ML, MH, CL, CH, and some
SM material, as defined in the Unified Soil Classification
System. It is noted that some of the embankment samples have a
low plasticity; i.e.,
plasticity index less than 10. The degree of compaction,
indicated by the samples tested, varied from about 78 to 86
percent of Standard Proctor
maximum density. These densities, though low, are not
inconsistent with what would be expected from construction
procedures such as apparently utilized in the dam. The pin
hole tests indicated the materials were not dispersive. Shear
strengths of the soils appear to be generally good despite the
relatively low densities. However, it must be noted that these
samples were, of necessity, taken from areas outside the
failure zone and must not be regarded as being fully
reoresentative of embankment strengths in the area of the
failure.

Physical Property Tests on Pipe. In an effort to attempt
to reconstruct the dam history, representatives of the Corps of
Engineers Construction Engineer Research Laboratory (CERL)
visited the site to obtain samples of various metal debris in
an attempt to date the pipe. Their findings are summarized as
follows:

Spiral riveted steel penstock pipe with flange
was installed in the early 1900's.

Pipe from the welded steel penstock to the power plant
is dated about 1940.